Automatic Trans . I have to shift myself on 1993 Toyota T100

Ok so my T100 is old 281967 miles on it
But I'd like to know. First off I manually shift this automatic transmission every where I go . Could it be a sensor , I've been doing this for 2 yrs now .Sometimes OD light blinks constantly
And when I get up to 55 it takes a looking time to shift into gear . Other times it shifts beautifully . Plz any ideas greatly appreciated :)